Apparatus for casting under low pressure is known, in which the crucible with the runner tube is disposed inside a hermetically sealed chamber directly under the plate which carries the casting mold. The drawbacks of these devices are that such an arrangement limits the capacity of the crucible; it is also impossible to provide ejectors and cores in the bottom half of the casting mold, since there is no place for the rejective actuating.
These drawbacks are not only a result of design considerations, but also of technology. The rise of the molten metal in the runner tube for filling the casting mold and its return result in oxidation along the walls of the runner tube which are wetted by the molten metal; the resulting oxide scale is carried into the casting mold during the next casting cycle. This can be avoided by the use of an inert gas instead of air, but this makes the process much more expensive.
The direct feeding of the casting mold from the crucible is disadvantageous because the variation of the level of the melt in it causes a variation of the velocity of filling the mold, which leads to an inconstancy of the quality of the produced castings.
The need for recharging of the crucible with molten metal, because of its limited volume, results in idling of the machine and reduces its productivity.
Low-pressure casting apparatus is known in which the crucible for the molten metal is disposed outside the device, which makes it possible to use crucibles of greater volume and to free space under the mold to receive different mechanisms. However, the ability to protect the metal from oxidation and absorbtion of gases from the air are greatly impaired.
The aforementioned drawbacks are typical also for known counterpressure casting apparatus.